Anaplasma are a genus of gram-negative bacteria that are obligate intracellular pathogens capable of infecting granulocytes, platelets and erythrocytes in vertebrate hosts. Anaplasma bacteria are transmitted to hosts through arthropod vectors, particularly various species of ticks. A. phagocytophilum infects neutrophils and causes anaplasmosis in mammals, including humans. The incidence of human granulocytotropic (or granulocytic) anaplasmosis (HGA, formerly known as human granulocytotropic ehrlichiosis) has increased steadily, from 1.4 cases per million persons in 2000 to 6.1 cases per million persons in 2010. A. phagocytophilum is transmitted primarily by Ixodes spp. of ticks. Because these Ixodes species ticks also transmit Borrelia burgdorferi (the causative agent of Lyme disease), simultaneous infection with A. phagocytophilum and B. burgdorferi is common.
A. platys causes infectious cyclic thrombocytopenia by infecting platelets and is thought to be transmitted by Rhipicephalus and Dermacentor spp. ticks. Although dogs are the most common host for A. platys infection, infection in other mammals, including cats, impalas, and sheep, have been reported. Co-infection of A. platys and E. canis due to the common vector of transmission has been known to occur.
Indirect immunofluorescence assays (IFA) and enzyme-linked immunosorbent assays (ELISA) have typically been used to detect Anaplasma infection. These assays detect the binding of anti-Anaplasma antibodies from a subject's blood, plasma, or serum to infected cells, cell lysates, or partially purified whole Anaplasma proteins. However, these assays for detecting anti-Anaplasma antibodies are limited in usefulness because of sensitivity and specificity issues directly related to the nature of the Anaplasma antigens used in these tests. Although polymerase chain reaction (PCR)-based tests with improved specificity and sensitivity have been developed, there is a continued need in the art for additional sensitive and specific assays for detecting Anaplasma antigens and serodiagnosis of anaplasmosis.